For several years Mr Zhou had oversight of China's internal security apparatus
China's former security chief Zhou Yongkang is being investigated for suspected "serious disciplinary violation", state media say.
The news confirms rumours about the hugely powerful former minister, who has not been seen in public for months.
Mr Zhou headed China's Ministry of Public Security and was a member of the top decision-making body, the Politburo Standing Committee.
The probe marks the first time such a high-ranking figure has been targeted.
Mr Zhou retired in late 2012 after a career that saw him head both the ministry charged with overseeing domestic security and China's largest energy company, the China National Petroleum Corporation (CNPC).
Several individuals believed to have had close ties to Mr Zhou have also been targeted for investigation in recent months.
